The present invention is directed to an equalizer that substantially eliminates ghosts in signals processed by a receiver.
Ghosts are produced in a receiver usually because a signal arrives at the receiver through different transmission paths. For example, in a system having a single transmitter, the multipath transmission of a signal may occur because of signal reflection. That is, the receiver receives a transmitted signal and one or more reflections of the transmitted signal. As another example, the multipath transmission of a signal may occur in a system having plural transmitters that transmit signals to the same receiver using the same carrier frequency. A network which supports this type of transmission is typically referred to as a single frequency network.
When a signal reaches a receiver through two or more different transmission paths, an interference pattern results. In the frequency domain, this interference pattern is manifested by a variable signal amplitude along the frequency axis. The worst case interference pattern results when the ghost is 100% and is shown in FIG. 1. This interference pattern has amplitude nulls or near amplitude nulls at certain frequencies. Therefore, any information contained in the received signal at these frequencies is likely lost because the signal to noise ratio near these frequencies is below a usable threshold.
A variety of systems have been devised to deal with the problems caused by ghosts. For example, spread spectrum systems deal very adequately with the problem of a 100% ghost by spreading the transmitted data over substantial bandwidth. Accordingly, even though a 100% ghost means that some information may be lost at the frequencies corresponding to amplitude nulls, a data element can still be recovered because of the high probability that it was spread over frequencies which do not correspond to amplitude nulls. Unfortunately, the data rate R associated with spread spectrum systems is typically too low for many applications. (The data rate R is defined as the number of data bits per Hertz of channel bandwidth.)
It is also known to use a matched filter in a receiver in order to deal with the problem of a ghost. In this approach, data is transmitted as a data vector. The matched filter correlates the received data with reference vectors corresponding to the possible data vectors that can be transmitted. Correlation of the received signal to the reference vector corresponding to the transmitted data vector produces a large peak, and correlation of the received signal to the other possible reference vectors produces small peaks. Accordingly, the transmitted data vector can be easily determined in the receiver. Unfortunately, the data rate R typically associated with the use of matched filters is still too low for many applications.
When high data rates, such as Rxe2x89xa71, are required, equalizers are often used in a receiver in order to reduce ghosts. A classic example of a time domain equalizer is an FIR filter. An FIR filter convolves its response h(t), shown generally in FIG. 2, with the received signal and produces a large peak representative of the main received signal. Ghosts have small components in the output of the FIR filter. However, as shown in FIG. 2, the values a1, a2, a3, . . . of the taps of an FIR filter depend on the value of a and, in order to perfectly cancel a 100% ghost using an FIR filter, the value a of the FIR filter response must approach 1. As the value a approaches 1, the values of the taps of the FIR filter do not asymptotically decrease toward zero. Therefore, the FIR filter becomes infinitely long if a 100% ghost is to be eliminated, making the FIR filter impractical to eliminate a 100% ghost.
Also, another problem with the use of an FIR filter is noise enhancement. If the transmitted signal picks up noise Nc in the channel, this noise is enhanced by the FIR filter so that the noise N0 at the output of the FIR filter is greater than the channel noise Nc. Also, if the channel noise Nc is white, the noise N0 at the output of the FIR filter is non-white, i.e., bursty.
An example of a frequency domain equalizer 10 is shown in FIG. 3. The frequency domain equalizer 10 includes a Fast Fourier Transform (FFT) module 12 which performs a Fast Fourier Transform on the received signal in order to transform the received signal to the frequency domain. A multiplier 14 multiplies the frequency domain output of the FFT module 12 by a compensation vector which includes a row of coefficients bi. An inverse FFT module 16 performs an inverse FFT on the multiplication results from the multiplier 14 in order to transform the multiplication results to the time domain.
It should be noted that, when the frequency domain equalizer 10 is used to eliminate ghosts, the frequency domain equalizer 10 must be included in every receiver. In order to reduce receiver cost, therefore, it is known to incorporate the inverse FFT module 16 into the transmitter so that the receivers require only the FFT module 12 and the multiplier 14. A consequence of moving the inverse FFT 16 to the transmitter is that data is transmitted in many discrete frequency channels. Accordingly, in the presence of a 100% ghost, the transmitted data is not recoverable around the null frequencies of FIG. 1.
FIG. 4 illustrates an exemplary set of coefficients bi which may be used by the frequency domain equalizer 10. In order to derive the coefficients bi, an estimator may be used at the output of the Fast Fourier Transform (FFT) module 12. This estimator models FIG. 1 and inverts this model in order to produce the coefficients bi of FIG. 4. Accordingly, the coefficients bi are chosen so that, when they and the FFT of the received signal are multiplied by the multiplier 14, the coefficients bi cancel the ghost. It should be noted that the coefficients bi should have infinite amplitudes at the frequencies where the interference pattern has a zero amplitude. However, the coefficients bi cannot be made infinite as a practical matter. Accordingly, the coefficients bi are cut off at these frequencies. An advantage of cutting off the coefficients bi is that noise enhancement at the frequencies where the coefficients bi are cut off is materially reduced. Thus, noise enhancement is lower at the output of the frequency domain equalizer 10 than would otherwise be the case. However, a disadvantage of cutting off the coefficients bi is that information in the received signal is lost at the cut off frequencies so that the output of the inverse FFT module 16 becomes only an approximation of the transmitted data.
Moreover, it is known to use empty guard intervals between the vectors employed in the frequency domain equalizer 10 of FIG. 3. The guard intervals are shown in FIG. 5 and are provided so that received vectors and ghosts of the received vectors do not overlap because such an overlap could otherwise cause intersymbol interference. Thus, the guard intervals should be at least as long as the expected ghosts. It is also known to use cyclic extensions of the vectors in order to give the received signal an appearance of periodicity. Accordingly, a Fast Fourier Transform of the received signal and a Fourier Transform of the received signal appear identical.
The present invention is directed to an equalizer which overcomes one or more of the above noted problems.
In accordance with one aspect of the present invention, a receiver receives a signal containing data distributed in both time and frequency. The receiver comprises a vector transform and a vector adjuster. The vector transform is arranged to perform a transform on the received signal using a plurality of transform vectors. The vector adjuster is responsive to the transform of the received signal in order to adjust the transform vectors so that the data can be recovered even in the presence of a strong ghost.
In accordance with another aspect of the present invention, a receiver receives a signal containing data distributed in both time and frequency. The receiver includes a vector transform that is arranged to perform a transform on the received signal using a plurality of receiver transform vectors. The receiver transform vectors are based upon a corresponding plurality of transmitter vectors and channel effects so that the data can be recovered by the vector transform even in the presence of a strong ghost.
In accordance with yet another aspect of the present invention, a receiver receives a signal from a channel. C* designates the channel with interference. The signal contains data, and the data has been processed by a transmitter transform so that the data is distributed in both time and frequency. A designates the transmitter transform. The receiver includes a receiver transform arranged to perform a transform on the received signal using a plurality of receiver transform vectors so as to recover the data even in the presence of a strong ghost, and T* designates the receiver transform. The receiver transform vectors are arranged so that the following equation is satisfied: Axc3x97C*xc3x97T*=I, wherein I is substantially the identity matrix.
In accordance with yet another aspect of the present invention, a communication system includes a transmitter and a receiver. The transmitter includes a transmitter transform A arranged to randomly distribute data to be transmitted in both time and frequency, and the transmitter is arranged to transmit a signal including the distributed data into a channel. The channel with interference is represented by C*. The receiver is arranged to receive the signal, and the receiver includes a receiver transform T* arranged to perform a transform on the received signal so as to recover the data even in the presence of a strong ghost. The receiver transform is arranged so that the following equation is satisfied: Axc3x97C*xc3x97T*=I, and I is substantially the identity matrix.
In accordance with a further aspect of the present invention, a transmitter includes a transmitter transform which is arranged to randomly distribute data to be transmitted in both time and frequency. The transmitter is arranged to add a guard interval to the randomly distributed data. The guard interval is known, is non-empty, and is non-related to the randomly distributed data. The transmitter is arranged to transmit the randomly distributed data and guard interval.